1. Field of the Invention
The present invention relates to video cameras and monitors in general and in particular to a novel interface circuit for displaying a video image from a video camera on a video monitor.
2. Description of the Prior Art
A television receiver or other video display device of the type that is normally used for displaying the output of a video camera is driven by horizontal and vertical sync signals having a predetermined frequency for controlling horizontal and vertical scan rates, respectively. In the United States, the predetermined frequencies of the horizontal and vertical sync signals are fixed according to an industry standard called an NSTC standard. In Europe, the corresponding frequencies are fixed according to another industry standard called a PAL standard.
A video monitor of the type that is commonly used as a peripheral device for displaying output data from a computer or the like is also driven by horizontal and vertical sync signals having a predetermined frequency. As in television receivers, the frequency of the horizontal and vertical sync signals control the horizontal and vertical scan rates. However, unlike the makers of television receivers, the manufacturers of video monitors are not required by any industry standard to use any particular frequency for the horizontal or vertical sync signals. As a consequence, most monitors made by different manufacturers and, indeed, different monitors made by a single manufacturer are found to use different frequencies for their horizontal sync signals.
While different frequencies are used for horizontal sync signals in video monitors, it is found that many manufacturers of video monitors use the same frequency for their vertical sync signals. That frequency, which typically corresponds to the frequency used by utility companies for generating electrical power, also happens to correspond to the frequency standard for NSTC and PAL vertical sync signals. In the United States that frequency is typically 60 Hz. In Europe that frequency is typically 50 Hz. It is also found that as long as the frequency of the vertical sync signals of a video monitor is identical to the frequency of the vertical sync signals of a video camera, the output of the video camera can be displayed on the monitor.